Volume scatter makes the render image and render view entirely black
When you give your environment a Volume Scatter shader, you need to use a light source other than a Sun Lamp, like a Point Lamp or a Plane with an Emission shader. A Sun Lamp essentially lights a scene from an infinite distance away. The light has to go through an infinite amount of space before it reaches your objects in your scene, and so a Volume density of anything greater then 0 will completely block the Sun's rays, since the Sun is an infinite distance away. So you have to use a light source that has a definite location, like a Point Lamp, Spotlight, or Emission shader. 

If you must use a Sun Lamp as your light source, instead of giving your whole environment a Volume Scatter shader, create a Cube or Sphere and give that a Volume Scatter, and just scale it up so it encompasses the visible area of your scene. That way a Sun Lamp can penetrate through the fog, since it only has a finite distance the light has to go through.